Studies with the murine mammary tumor model have established the feasibility of using plasma-concentrations of viral protein to assess the presence and status of solid tumor. The viral protein utilized for such studies was a 52,000 dalton glycoprotein (gp 52) isolated from the murine mammary tumor virus (MMTV) using affinity chromatography. Availability of purified gp 52 by the aforesaid procedure allowed development of a radioimmunoassay to this protein sensitive to plasma levels down to 0.1 ng/100 .mu.l. See in regard to the above Ritzi et al., Virology, 75, 188 (1976) and Ritzi et al., Proc. Natl. Acad. Sci. USA, 73, No. 11, 4190 (1976).
The relationship between the mammary tumors and the plasma levels of gp 52 were found to be as follows:
(a) tumor-bearing mice, male or female, showed markedly elevated (100-1000 ng/ml) levels of gp 52 as a free soluble protein in the plasma and the mean concentration increased with average tumor size;
(b) the presence of another malignancy (leukemia) did not result in any change of gp 52 levels in the plasma;
(c) mammary tumor tissue located by transplantation outside the mammary gland is also detected by high plasma gp 52 levels;
(d) low (2-10 ng/ml) plasma levels of gp 52 are found in tumor free mice, whether they originate from strains characterized by high or low frequencies of spontaneous mammary tumors;
(e) tumor-free lactating females exhibit the normally low levels of plasma gp 52 despite the fact that their milk contains an average of 20,000 ng/ml of this protein; and
(f) the circulatory clearance time of gp 52 in tumorous animals is sufficiently rapid (a half-life of 4-6 hr.) to suggest a requirement for continued replenishment to maintain the high levels observed.
The MMTV model provides the basis for establishing the feasibility of utilizing a viral protein "marker" in plasma for monitoring the presence and status of human breast cancer. The use of a viral protein marker would be distinguishable from assays, such as disclosed in U.S. Pat. No. 3,999,944, which rely on detecting antigen induced leukocyte adherence inhibition caused by tumor specific cell mediated immunity.